It's no secret that Apple has a new manufacturing plant in Mesa, where the company expects GT Advanced Technologies Inc. to make high-tech glass for its devices.

But what is going on behind the walls of the former First Solar plant at 3740 S. Signal Butte Road is a big secret.

Apple Inc., the Silicon Valley tech giant that just spent $3 billion to buy the Beats headphone and music company, is notoriously unresponsive to simple inquires about its operations. This despite $10 million in taxpayer funds the Arizona Commerce Authority has promised it.

Apparently, this does not buy any transparency from Apple.

Here is what we do know. Apple bought the failed First Solar plant in 2013 for $113 million with plans to lease the 1.3 million-square-foot facility to GT Advanced Technologies.

GTAT, based in New Hampshire, is gearing up to make scratch-resistant sapphire glass for Apple, but it's unclear where the company is in that process.

GTAT is leasing the manufacturing plant from Apple.

"There's not much we can talk about publicly," GTAT spokesman Jeff Nestel-Patt said of the Mesa operation. "It's not a topic that we can talk at length about. I've just about used up my whole vocabulary on the subject."

Nestel-Patt was apologetic on the phone, explaining that GTAT has a confidentiality agreement with Apple.

Apple did not return calls.

In its first-quarter earnings report last month, GTAT President Tom Gutierrez said his company has received prepayments from Apple for its Mesa project.